When off-hand placing blocks and mining them at the same time (such as in a concrete converter) unbreaking 3 affects the mining speed.

If the pickaxe used to mine the blocks is enchanted with just efficiency 5 one block can consistently be placed and mined every 8 game ticks. If, however, the pickaxe is additionally enchanted with unbreaking 3 the placing and mining of blocks becomes 1) faster than without unbreaking 3 although unbreaking should have no effect on mining speed and 2) visually "jittery"/irregular.

See this YouTube video (unlisted) for a demonstration of this issue.